Answers

From your description, you want to redirect to the URL 2 or URL 3 when user access the URL 1.
We can use HTTP Module to achieve it.

The following steps for your reference:
1.Create a HTTP Module using visual studio.

2.Create a Class implements IHttpModule interface and check user’s IP (intranet IP or external IP) in the
Application_BeginRequest method.If the IP from intranet, redirect to URL2. If the IP from external, redirect to URL3.